The use of polyvinyl chloride (PVC)-based membranes as a commercial roofing material has come into widespread application in the roofing industry inasmuch as the material is flexible, waterproof and may be rapidly applied over a large area as new roofing, re-roofing or over existing roofing by a wide variety of methods, including mechanical attachment or gluing, with adjacent panels being joined by seams formed by heat-welding or solvent-welding. Such PVC-based roofing membranes typically comprise a three-ply composite of a layer of a polyester, e.g., polyethylene terephthalate (PET) or fiberglass fabric reinforcement sandwiched between two PVC films, the overall composite typically being 30 to 90 mils thick.
In the late 1990's waterproof flexible photovoltaic solar panels were developed that could be secured to roofing materials, including the aforesaid composite reinforced PVC membranes, thereby permitting the passive generation of electrical energy from rooftops by exposure to the sun, in essence giving roofs a dual utility of conventional protection from the elements and the generation of power.
A fundamental problem with securing such flexible photovoltaic solar panels to PVC roofing membranes lies in the incompatibility between (1) the adhesive typically used to glue the solar panels to the roofing membranes and (2) the conventional plasticizers used in PVC roofing membranes. More specifically, the PVC in the PVC roofing membrane contains certain plasticizers to enhance the membrane's flexibility, and the most widely used adhesive is a butyl rubber-containing asphalt adhesive either applied to the back side of the solar panels or in the form of two-sided tape. Over a period of two to four years the conventional plasticizers in the PVC layers of the reinforced PVC roofing membrane migrate from the PVC into the butyl rubber-containing asphalt adhesive, which softens the adhesive bond and compromises the elasticity of the PVC layer of the roofing membrane, which in turn weakens and/or destroys the bond between the solar panel and the roofing membrane and seriously compromises the flexibility that allows for expansion and contraction of both the PVC roofing membrane and the flexible solar panel. Although other classes of adhesives have been investigated for securing such solar panels to such PVC roofing membranes, they have more drawbacks than such a butyl-based adhesive. For example, acrylics lack sufficient bond flexibility as soon as they have been applied and are costly, while ethylene vinyl acetate hot melts require special dedicated factory equipment and narrow temperature ranges, making them essentially useless on a job site.
Accordingly, there is a need in the art for a method of securing conventional flexible solar panels to PVC roofing membranes that provides a strong, elastic, long-lasting bond between the two. This need is met by the present invention, which is summarized and described in detail below.